1. Field of the Invention
Generally, the present disclosure relates to the field of fabrication of integrated circuits, and, more particularly, to semiconductor devices having conductive lines, such as gate electrodes of field effect transistors, metal lines in the wiring levels of a semiconductor device, which are formed on the basis of advanced photolithography techniques.
2. Description of the Related Art
The fabrication of microstructures, such as integrated circuits, requires tiny regions of precisely controlled size to be formed in a material layer of an appropriate substrate, such as a silicon substrate, a silicon-on-insulator (SOI) substrate, or other suitable carrier materials. These tiny regions of precisely controlled size are generated by patterning the material layer by performing lithography, etch, implantation, deposition processes and the like, wherein typically, at least in a certain stage of the patterning process, a mask layer may be formed over the material layer to be treated to define these tiny regions. Generally, a mask layer may consist of or may be formed by means of a layer of photoresist that is patterned by a lithographic process, typically a photolithography process. During the photolithography process, the resist may be spin-coated onto the substrate surface and then selectively exposed to ultra-violet radiation through a corresponding lithography mask, such as a reticle, thereby imaging the reticle pattern into the resist layer to form a latent image therein. After developing the photoresist, depending on the type of resist, positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the layer of photoresist. Based on this resist pattern, actual device patterns may be formed by further manufacturing processes, such as etch, implantation, anneal processes and the like.
Since the dimensions of the patterns in sophisticated integrated microstructure devices are steadily decreasing, the equipment used for patterning device features have to meet very stringent requirements with regard to resolution and overlay accuracy of the involved fabrication processes. In this respect, resolution is considered as a measure for specifying the consistent ability to print minimum size images under conditions of predefined manufacturing variations. One important factor in improving the resolution is represented by the lithographic process, in which patterns contained in the photomask or reticle are optically transferred to the substrate via an optical imaging system. Therefore, great efforts are made to steadily improve optical properties of the lithographic system, such as numerical aperture, depth of focus and wavelength of the light source used.
The resolution of the optical patterning process may, therefore, significantly depend on the imaging capability of the equipment used, the photoresist materials for the specified exposure wavelength and the target critical dimensions of the device features to be formed in the device level under consideration. For example, gate electrodes of field effect transistors, which represent an important component of modern logic devices, may be 50 nm and even less for currently produced devices, with significantly reduced dimensions for device generations that are currently under development. Similarly, the line width of metal lines provided in the plurality of wiring levels or metallization layers may also have to adapted to the reduced feature sizes in the device layer in order to account for the increased packing density. Consequently, the actual feature dimensions may be well below the wavelength of currently used light sources provided in current lithography systems. For example, in critical lithography steps, an exposure wavelength of 193 nm may be used, which therefore may require complex techniques for finally obtaining resist features having dimensions well below the exposure wavelength. Thus, highly non-linear processes are typically used to obtain dimensions well below the optical resolution. For example, extremely non-linear photoresist materials may be used, in which a desired photochemical reaction may be initiated on the basis of a well-defined threshold so that weakly exposed areas may not substantially change at all, while areas having exceeded the threshold may exhibit a significant variation of their chemical stability with respect to a subsequent development process.
The usage of highly non-linear imaging processes may significantly extend the capability for enhancing the resolution for available lithography tools and resist materials. However, the increased non-linearity may result in significant distortions of the resulting device features compared to the original features provided on the reticle. Lithography processes with low linearity generally suffer, among others, from large line-end pullback, significant corner rounding, a strong dependence between feature area and printed CD (critical dimension), large CD vs. pitch variations.
FIG. 1a schematically illustrates a typical feature 151 as provided on a reticle 150 compared to a representation 161 of the actual device feature after imaging the feature onto a resist layer by using a highly non-linear imaging process. As shown, the design feature 151 formed on the reticle 150 may include line-like segments 152 having respective end portions 152c. The respective design dimensions, such as a width 152w and a length 152l, may depend on the technology node used. As previously explained, the width 152w, which may represent the width of a metal line when the feature 151 is designed for a metallization layer of a semiconductor device, or may represent the length of a gate electrode, and the like, may define the final performance of the circuit feature under consideration. The same holds true for the length 152l or the degree of corner rounding, depending on the specifics of the circuit layout of interest. Furthermore, the neighborhood of the feature 151, as well as the local process conditions, may also significantly affect the imaging process. Thus, the finally obtained result of the imaging process may locally vary on the wafer with respect to the local conditions prevailing during the imaging process and the layout of the reticle in the vicinity of the feature 151.
The reticle 150 may typically be comprised of an opaque material, such as chromium, possibly in combination with other materials, that is formed on an appropriate substrate material, which is substantially transmissive for the exposure wavelength under consideration. In other cases, the reticle may represent a reflective mask, wherein the reflectivity is appropriately modified so as to represent the desired features 151. The features 151 are formed by advanced mask print techniques based on optical lithography, electron beam lithography and the like.
The reticle 150 may then be used as a mask during an optical lithography process to obtain a latent image in a corresponding resist material that is provided on a substrate, such as a semiconductor wafer, on the basis of complex pre- and post-exposure treatments. The latent image in the resist material is then developed to form a resist feature that may correspond to the feature 151, or at least a perform thereof, if further trim processes may be required for further reducing the dimensions of the resist feature prior to actually forming a permanent device feature on the basis of the resist feature. For convenience, it may be assumed that the device feature 161 may represent the finally obtained feature on the basis of the design feature 151 in combination with a highly non-linear imaging process. As shown, a significant degree of corner rounding may be obtained and also a “pull back” of the line ends may be observed. That is, the end portions 152e may be rounded and may be “withdrawn” compared to the desired design dimensions. It should be appreciated that the dimensions of the feature 161 formed on a substrate 161 may generally be reduced by the projection factor defined by the optical projection system used for performing the imaging process. For example, for a desired width 162w of 100 nm, the corresponding width 152w may be approximately 500 nm, when the imaging optics provides a reduction factor of 5. However, due to the respective distortion of the feature 161 relative to the design feature 151, significant corrections are usually provided on the reticle side in order to reduce the non-linearity. Such corrections upon reticle design may be referred to as optical proximity corrections (OPC), which may have to performed more aggressively with an increasing degree of non-linearity of the imaging process. Consequently, OPC corrections used to compensate for undesired pattern deformation in processes of high non-linearity must simultaneously provide significant yet highly precise corrections to the mask design of the reticle 150. These corrections may be performed on the basis of complex OPC models, which may locally simulate the imaging process for a given pattern in order to determine appropriate corrections for the mask layout of the pattern under consideration to remove or reduce the undesired effects of the non-linear imaging process. Due to the highly local nature of the non-linearity effects, great efforts are required in terms of computational resources and the capacity of the design databases for performing OPC corrections and redesigning the corresponding reticles on the basis of the corrections. In many cases, however, the corrections provided by the OPC tool may themselves result in non-desired modifications of the finally obtained device features.
FIG. 1b schematically illustrates the feature 151 formed on the reticle 150 on the basis of OPC corrections 153, which are illustrated in dashed lines. For example, respective modified end portions 152M may provide a reduction of the undesired pull back of the end portions 151e. The resulting features 161 may have a size and shape that substantially corresponds to the design values. However, due to limitations of the OPC model, such as restricted design databases, computational resources or other process conditions that may not be precisely incorporated into the respective model, the corrections may locally provide faulty device features. For instance, the modified end portions 152M may result in a corresponding short between opposing line end portions, which cause severe performance degradations or even a complete failure of the device under consideration.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.